1. Field of the Invention
The present invention relates to a high performance side stream infrared gas analyzer which continuously determines the concentration of predetermined constituents (e.g., CO.sub.2 and N.sub.2 O) of the respiratory gases of a patient. In particular, the present invention relates to a side stream infrared gas analyzer which can be mounted directly onto a printed circuit board without additional mechanical components and which is configured such that the infrared transmissive windows of the detector and the infrared source also serve as respective windows on the sample cell for containing the gas stream for analysis.
2. Description of the Prior Art
It is frequently of critical importance to monitor the concentrations of carbon dioxide (CO.sub.2) and nitrous oxide (N.sub.2 O) in the gases inspired and expired from a patient under anesthesia, for expired CO.sub.2 and N.sub.2 O concentrations are reliable indicators of the carbon dioxide and nitrous oxide concentrations in the arterial blood. In a clinical setting, monitoring expired CO.sub.2 and N.sub.2 O prevents malfunctions in anesthesia rebreathing apparatus from going undetected and delivering excessive amounts of CO.sub.2 or N.sub.2 O to the patient. Rebreathing of anesthetic gases is very cost effective and environmentally desirable, but accurate CO.sub.2 and N.sub.2 O concentrations are difficult to maintain in the patient's bloodstream without a concentration monitor.
It is known by those skilled in the art that directing infrared radiation through a sample of a gaseous mixture and measuring the incident radiation illuminating a detecting device on the other side of the sample will provide a measure of the infrared absorption of the gas. Electrical signals produced by such a detecting device are indicative of the infrared absorption of the gas and can be processed to produce an output indicating the concentration of one or more of the constituents of the gas being analyzed. This type of gas analyzer operates on the principle that various gases exhibit substantially increased absorption characteristics at specific wavelengths in the infrared spectrum and that higher gas concentrations exhibit proportionally greater absorption.
Infrared respiratory gas analyzers for use in critical care applications come in two pneumatic configurations, namely, mainstream and side stream. A mainstream analyzer is placed in the patient's respiratory circuit and measures the absorption of infrared light transmitted through the patient's inspired and expired respiratory gases as they flow through the respiratory circuit. Such a mainstream infrared gas analyzer is described in detail by Yelderman et al. in U.S. Pat. Nos. 5,081,998, 5,095,913 and 5,281,817 assigned to the present Assignee and hereby incorporated by reference in their entirety. These applications describe an infrared detector and a shutterless optically stabilized capnograph which has no moving parts, which does not require a modulated source of infrared radiation, and which does not suffer from thermal drift. The disclosed infrared detector includes a substantially identical pair of thermopile detectors mounted on the same ceramic substrate and connected in series opposition. Because of this configuration, balanced and equal incident radiation illuminating the pair will produce no signal. Also, because the reference junctions of both detectors are on the same ceramic substrate and at substantially the same temperature, a drift in substrate temperature will produce no discernible change in output signal. In order to make the system respond to incident radiation, a blocking filter is placed over one of the thermopile detectors in the pair. With the filter in place, the system responds to incident radiation but is substantially insensitive to other thermal changes since the effect of a variation in background signals is compensated by subtracting the outputs of the two thermopile detectors. Conventional side stream analyzers, on the other hand, draw a small, continuous sample of the respiratory gases through a fixed sample cell and out through an exhaust port of the sample cell. The side stream analyzer measures the absorption of infrared light as it is transmitted through the sample cell. Typically, a side stream analyzer requires a pneumatic sample system which incorporates pumps, tubing and fittings. The sample system may also require valves, flow controls, pressure controls and moisture filters or separation devices. For example, a simple configuration which uses a pump to supply the sample gas to the sample cell is illustrated by Passaro et al. in U.S. Pat. No. 4,692,621. Since conventional mainstream infrared gas analyzer configurations take advantage of the primary flow of the respiratory gases, they do not require the additional complexity of a pneumatic system such as those used in prior art side stream infrared gas analyzer configurations.
A mainstream infrared gas analyzer of the type described in the aforementioned Yelderman et al. patents requires the optical and electronic components to be physically connected to the patient's airway or respiratory circuit. As a result, a mainstream gas analyzer may be subjected to mechanical abuse and temperature variations when in use. A side stream configuration, on the other hand, allows the optical components to be remotely located from the patient's respiratory circuit so that the optical and electronic components (i.e., the optical bench) can be protected by a fixed, temperature controlled housing. Thus, while a mainstream configuration has the advantage of reduced complexity, side stream configurations are often desired since they have the advantage of protection from damage and thermal gradients.
Conventional side stream optical benches use infrared detectors which must be stabilized by mechanical chopping techniques. As just noted, conventional side stream optical benches also require accurate temperature control of the detector environment to assure stability. For example, such a conventional side stream infrared gas analyzer is disclosed by McClatchie et al. in U.S. Pat. No. 4,177,381. McClatchie et al. therein describe an infrared gas analyzer which utilizes mechanical choppers and temperature controllers in their measurements. McClatchie et al. also utilize a sample cell which directs the air therein so as to prevent direct impingement of oils, particulate matter, and other contaminants onto the infrared transparent windows so as to prevent contamination of the windows. Unfortunately, this system is quite complex and expensive and relatively unreliable because of the numerous mechanical elements.
A simpler, more reliable side stream gas analyzer has been developed by the present inventors. Prior art FIG. 1 illustrates a cutaway view of a side stream infrared gas analyzer 100 of the type described in U.S. Pat. No. 5,282,473, also assigned to the present Assignee and hereby incorporated by reference in its entirety. The prior art side stream infrared gas analyzer 100 includes a sample cell 101, an infrared source (not shown), and an infrared detector 102 mounted in opposite housing halves 104 so that their optical axes are aligned with respect to the sample cell 101. As shown, housing halves 104 include a clearance 106 for accommodating the infrared source and a separate cavity 108 for accommodating the infrared detector 102. As illustrated, the infrared detector 102 may comprise a plurality of constituent selective filters 109 in a plurality of constituent channels including a reference channel. Optical funnels 110 and 112 are included in the respective housing halves 104 to reduce the optical apertures of the infrared source and the infrared detector 102 at the windows 114 and 116 disposed on ledges 118. The walls of the optical funnels 110 and 112 are treated by plating or painting a thin layer of gold over a nickel plating so that the optical funnels 110 and 112 are highly reflective at the infrared wavelengths output by the infrared source. In addition, the gas passageway into the sample cell 101 is shaped to create smooth transitions from the round cross-section at the gas tube inlet connection from the patient's airway to the rectangular cross-section at the aperture 120 where the gas passageway intersects the optical path to define the detection volume within the sample cell 101.
While the side stream gas analyzer 100 of prior art FIG. 1 significantly decreases the optical path length and significantly improves the response time and signal levels of the device, a smaller and less cumbersome side stream gas analyzer is desired which does not require optical funnels 110 and 112 and which uses even less electrical power. Ideally, such a side stream gas analyzer is configured so that it is small and compact enough to be mounted directly onto a printed circuit board without requiring additional mechanical components. The present invention has been designed to meet these needs in the art.